OK I got it.
Weight 4 with other 4, that's the first one.
Take the heavier side, and weight with the other 4 left, that's the second one.
IF they weight the same, that means there is a light one on the 4 bbs left.
IF they weight differently, that means there is a heavier one on the heavier side.
Now you take those 4 different bbs and weight than, 2 each side, that's the third one.
Take the 2 who were heavier or lighter (depends on what happened on the second try) and put than on the balance, you'll get the different one, and... that's the fourth weighing. There you go.